There exists a class of devices for accessing fluid spaces and vessels within a human (or animal) body that are generally referred to as "ports". Herein, "vessel" is defined as any conduit carrying a fluid within the patient's body. These prior art devices comprise a chamber having an access opening sealed by means of a septum and having an egress from a second location leading to a catheter disposed within a fluid space or vessel. The septum allows a needle to pass into the chamber, but then closes when the needle is removed, thereby preventing fluid leakage from within a space or vessel and also preventing anything from entering or exiting the chamber. These devices are usually implanted below the skin to prevent infection, other contamination, and mishandling.
Ports are designed for relatively infrequent use, perhaps once a week, and, importantly, for flowrates of 50 milliliters per minute or less, as is common during chemotherapeutic treatment. Modification of these devices for hemodialysis, plasmapheresis, and other fluid exchange therapies, which require much greater flowrates, by simply enlarging the device components, poses several serious drawbacks that effectively limit use in such applications. First, the septum degrades quickly due to the larger gauge needles necessary to accommodate the flowrates required in hemodialysis. Repeated puncturing of the septum by these large needles produces numerous free-floating septum fragments that can find their way into the circulatory system. Accordingly, the useful life of the devices is substantially shortened, thereby defeating one of the purposes of using an implanted subcutaneous device. Second, the flowpath has several stagnation points where clots may form and also is not completely flushable or easily cleaned, thereby providing breeding grounds for infection, once contaminated, or a build-up of material, which may adversely affect function. Third, the flowpath is not streamlined and contains flowpath obstructions, sharp corners, and abrupt changes in flow area and flow direction. This tends to increase the shear stress and turbulences experienced by blood flowing through the device due to the significantly higher flowrates required in hemodialysis, thereby increasing erythrocyte damage and platelet activation. Also, the tortuous flowpath increases the pressure drop through the devices, which can increase air release and foaming, causing the dialysis machine's safety alarms to activate.
Typical access port apparati are disclosed in U.S. Pat. Nos. 5,180,365 (Jan. 19, 1993), 5,226,879 (Jul. 13, 1993), 5,263,930 (Nov. 23, 1993), and 5,281,199 (the '199 patent) (Jan. 25, 1994); all entitled "IMPLANTABLE ACCESS DEVICES" and all issued to William D. Ensminger as either the sole or the first-named inventor. The following discussion concerns the '199 patent; while all of the references are relevant, the '199 patent embodies the most recent material and also incorporates material from each of the earlier patents.
The devices described in the '199 patent include a funnel-shaped entrance to an access housing, which is fixed to the surrounding tissue. The housing is connected to an articulated valve, which is in turn joined to a catheter. Several types of valves are disclosed, including one that is a tube fabricated in a flattened shape that is forced open by the insertion of a filament. Other valves disclosed include manually activated types. In these manual valves, manual actuation applied to the skin and translated to the device moves two disks which slide over and in contact with each other to align holes in those disks. A needle may be inserted when the holes are aligned; the disks secure the needle in the housing when the external manual pressure is released. This patent also discloses a curved entry (presumably to allow the needle to enter at a convenient angle to the skin but still align parallel to the vessel). The disclosure of this patent, in column 9, line 53, mentions use in hemodialysis treatment.
The Ensminger et al. '199 device has several characteristics that lead to problems. First, in most embodiments the curved needle must be flexible, and as such can kink or of otherwise restrict flow. However, when the needle is inserted, no such kink can be seen by the operator, and may not be detected before damage to the patient results. Another drawback of these devices can best be seen by inspection of FIG. 1A of the '199 patent, showing an abrupt change in flow diameter where the catheter 46 is joined to the valve 24. Abrupt changes form space for fluid stagnation to occur and/or eddy currents that promote clot formation. Further, such spaces are not easily flushed due to the lack of a streamlined flowpath. This same problem is shown in FIG. 1A of this patent in the stagnant space around the leaf valves 38. Indeed, such problems exist at nearly every transition point between the various structures and assemblies of the '199 device.
A further drawback of the '199 device is the attachment of the housing to the surrounding tissue. Since the housing cannot move to accept a rigid needle, the needle must be closely aligned with the port entrance. Otherwise, the needle must be moved transversely under the skin causing discomfort for the patient. Ensminger et al. required the use of a flexible tube to solve this problem. A still further drawback of the '199 apparatus is shown in FIGS. 41-43. These drawings show needle points where the flow has a radial direction component as it leaves the needle. This change of direction, especially under high flowrates, can severely damage hematocytes and activate platelets. Also, the flexible tube will have a greater flow resistance and higher shear than a rigid straight needle having a similar outside diameter.
A general limitation in all relevant prior art devices is the lack of a streamlined flowpath. Without such streamlining, stagnant volumes exist where clots may form and shear stress is higher, tending towards erythrocytic damage. Such locations cannot be flushed or easily cleaned. Blood residue remaining in the devices after flushing may clot and provide breeding grounds for infection, once contaminated. In addition, pressure drops and abrupt flow direction changes may damage blood components.
The Ensminger '199 device is still further limited by its lack of effective sealing provisions. There is no force urging the valve to seal. The valve is therefore not fault-tolerant and particles, clots, skin fragments, and imperfections on the inside surface of the valve will cause leakage. In addition, the valve opens in response to very low changes in pressure. Further, the seal is in line with the housing, making the device longer and increasing the changes in pressure experienced by fluids passing through the device. Finally, there is no locking mechanism whereby the needle may be secured to the device.
Accordingly, it is an object of this invention to overcome the above illustrated inadequacies and problems of extant devices by providing a totally implantable access means suitable for repeated use in applications (e.g., hemodialysis) with blood flowrates of 250 milliliters per minute or more yet with low pressure drops along the flowpath.
It is another object of this invention to provide a laminar flowstream, even during flow diameter transitions.
It is a further object to provide means where the flowpath is streamlined and provides substantially no stagnation points, and also to provide an apparatus where the entire flowstream is flushable.
It is a still further object of this invention to provide apparatus suitable for single- and dual-lumen catheter systems.
It is yet another object of this invention to provide an access housing that is less painful during needle insertion and more accommodating during dialysis for the patient.
It is a further object to secure the needle within the access housing during the dialysis session.
It is another object of the invention, when using dual-lumen catheters to secure both needles to each other.
It is a still further object to have lower clotting, stenosis, and infection rates than synthetic grafts.
It is yet another object to have lower infection and lumen clotting than percutaneous catheters.